1. Field of the Invention
The present invention is in the field of integrated circuits. More particularly, the present invention relates to high density CMOS technology integrated circuits of the very large scale integrated circuit (VLSI) type, and of the ultra large scale integrated circuit (ULSI) type, which includes structure of highly thermally conductive material so positioned and arranged as to cooperate with the remainder of the integrated circuit structure, and also with a package of the integrated circuit, so as to conduct heat rapidly out of high heat generation areas of the integrated circuit, and directly to the package for dissipation into the ambient.
2. Related Technology
Historically, semiconductor processing and fabrication has changed from the NMOS technology used in the 1970's and 1980's, to CMOS technology used today. The driver for this technology change was power consumption (and the related heat dissipation) within the integrated circuit chips. Integrated circuits using CMOS technology have the advantage of similar electrical performance to the NMOS circuits, but at a lower power consumption and lower heat dissipation. Because of the lower heat dissipation, a less expensive package could be used with CMOS integrated circuits, which partially offset the initially higher processing costs for the fabrication of integrated circuits using the CMOS technology. The CMOS technology integrated circuits also have an advantage in improved reliability resulting from a lower operating temperature for the chips and their packages.
However, with the development of VLSI integrated circuits, it became apparent that the problem of marginal or inadequate heat dissipation and excessive operating temperatures had only been delayed, and not avoided entirely, by the change to CMOS technology. That is, the recent developments in VLSI CMOS integrated circuits of the high-density type have again brought forth serious problems with excessive operating temperatures and inadequate cooling. These problems go beyond the scope of cooling which can be provided by the use of mere conventional passive heat sinks attached to the integrated circuit package. Even in the usual use environment for such packaged integrated circuits in which a chassis cooling fan is used, the heat dissipation from some integrated circuits is inadequate, and excessively high and life-shortening temperatures are experienced.
One recent solution of a stop-gap nature has even involved mounting a small dedicated cooling fan directly to an integrated circuit chip mounting package. Another solution also of a stop-gap nature is to reduce the operating voltage of the integrated circuit chip from the traditional 5 volts to 3.3 volts, or even to 2.5 volts. However, the use of a dedicated chip-cooling fan places the life of an expensive integrated circuit at risk should the dedicated fan fail. The voltage-reduction expedients compromise the ability to easily interface the integrated circuit chip with other circuits designed to operate at the higher and traditional 5 volt level. At the ULSI levels of chip size and circuit density, cooling of integrated circuit chips has been recognized as a serious problem for which no adequate solution heretofore existed. The conventional cooling expedients of using special liquid-tight heat conductive packages, some of which provided spring loaded cooling fingers extending into contact with the integrated circuit chips themselves; and of providing for liquid cooling of the integrated circuits, such as is used in some large main-frame and super computers is not considered to be economically feasible for the lower cost and smaller systems in which many ULSI chips will be used.
In CMOS integrated circuits a significant portion of the dissipated heat is generated as the output drivers. Thus, these output driver circuits represent a particularly large part of the total heat generation within the integrated circuit chip. This heat generally spreads thorough and affects the entire chip. Moreover, the heat generated within a chip must be removed efficiently because virtually all failure mechanisms are enhanced by increased operating temperatures. Electromigration and oxide breakdown effects all worsen with increased temperature. Further, leakage currents increase in reverse-biased junctions and in turned-off MOS transistors with increased temperatures. At higher temperatures, corrosion mechanisms accelerate, and greater differential thermal expansion stresses are generated at material interfaces, such as at solder and wire-bond joints, as well as interfaces between semiconductor materials and metallic structures such as conductors of the integrated circuit.
With an integrated circuit chip in its package, radiation offers too small a contribution to the heat dissipation required to be considered a significant factor in cooling the integrated circuit. Conduction of heat within the chip from the areas of heat generation, and from the chip to the package for liberation into the ambient is the only effective mechanism for cooling the packaged integrated circuit chip. As mentioned above, cooling expedients, such as chassis fans and package-mounted fans may be used to liberate heat from the package to the ambient. The necessity for increased power (heat) dissipation from an integrated circuit chip increases with increased integration density, die size, and circuit speed. The ambient temperature is usually room temperature. Consequently, as the power dissipation from a chip increases, conductive heat transfer rates within the chip, from the chip to the package, within the package, and from the package to the ambient, must be addressed in order to maintain the temperatures within the chip at acceptable levels.
The following table provides an indication of the heat transfer conductivities of various materials used in the construction of integrated circuits and their packages.
______________________________________ Material Thermal conductivity (W/cm-.degree.K.) ______________________________________ Silver 4.3 Copper 4.0 Aluminum 2.3 Tungsten 1.7 Molybdenum 1.4 Silicon 1.5 Germanium 0.7 Gallium arsenide 0.5 Silicone carbide 2.2 Alumina 0.3 Silicon dioxide 0.01 ______________________________________
As can be seen from the above table, there is a considerable difference in the heat transfer coefficients of these various materials. These differing conductive heat transfer rates should be kept in mind as the following description of a preferred embodiment of the invention is described. A similar difference among the materials applies with respect to the thermal coefficient of expansion of these various materials.